A radioactive gas monitoring device is provided in the containment vessel of a pressurized-water nuclear power plant to detect leakage of the primary coolant. The primary coolant leaking into the containment vessel diffuses into the air in the vessel, and the air is sampled as a sampled gas into the sample chamber of the detector unit. Fission products (radioactive noble gases) contained in the sampled gas emit beta rays. The radioactive gas monitoring device detects the beta rays with a radiation detector.
“Guideline for Radiation Monitoring of Nuclear Power Plants (Japan Electric Association Guide (JEAG) 4606-2003)” has been stipulated as a Japanese national guideline regarding radiation monitoring. The radioactive gas monitoring device calculates using its measurement unit a count rate of detection signal pulses output from the detector. When the count rate becomes higher than a preset level, an alert is issued to give notice to operators and necessary system isolation is automatically performed by using the alert as an interlock signal.
Typical radioactive nuclei to be produced depend on the conditions of the fuel cladding tubes inserted in the nuclear reactor. Specifically, radioactive noble gases such as Xe-133, Xe-135, and Kr-85 are main measurement object in a case of a high fuel failure rate. The lower the fuel failure rate is, the more the concentration of nuclear fission products reduces, which leads a radioactive substance such as F-18 to manifest itself and become dominant. Generally, radioactive gas monitoring devices detect beta rays emitted from radioactive novel gases with a plastic scintillation detector (see Patent Documents 1 to 6, for example).